1. Field of the Invention
The present invention relates to a semiconductor device using a thin film transistor (TFT) mounted on an insulating substrate such as a glass plate, and more particularly to a semiconductor device which can be utilized in an active matrix type liquid crystal displaying unit, or the similar matrix circuit.
2. Description of the Related Art
An active matrix type liquid crystal display unit using a TFT to drive a pixel, an image sensor, a three dimensional integrated circuit, and the like are known as a semiconductor device having a TFT on an insulating substrate such as a grass plate.
A thin film silicon semiconductor is generally used as the TFT mounted on such a device. In particular, for a high speed operation it is strongly required to establish a method for manufacturing a TFT comprising a crystalline silicon semiconductor. A method of conducting crystallization by forming an amorphous semiconductor film and applying a heat energy thereto (heat annealing) is known as a method for obtaining such a crystalline thin film silicon semiconductor.
There are some problems in manufacturing a semiconductor circuit using the crystalline silicon film thus formed. For example, a circuit that not only a matrix circuit but also the peripheral circuit for driving the same are constituted of the TFT (monolithic type active matrix circuit) is taken into account as an active matrix type circuit used in a liquid crystal display unit (i.e., a circuit that a controlling transistor is arranged in each pixel).
In this complicated circuit, characteristics required in the TFT vary depending on the position of the circuit. For example, the TFT used for controlling the pixel of the active matrix circuit is required to have sufficiently small leak current in order to maintain an electric charge stored in a capacitor constituted of a pixel electrode and an opposite electrode. However, a current driving ability may not be so high.
On the other hand, a large current switching at a short time is necessary in the TFT used in a driver circuit which supplies signals to a matrix circuit, and the TFT having a high current driving ability is required. However, a leak current may not be so low.
A TFT having a high current driving ability and a low leak current is most desirable. However, the TFT presently manufactured is far from such an ideal TFT, and if the current driving ability is high, the leak current is also high, and if the leak current is low, the current driving ability is low.
Therefore, the monolithic type active matrix circuit constituted using the conventional TFT attempts to improve the current driving ability and reduce the leak current by changing a channel length or a channel width of the TFT. However, if the circuit becomes finer, the change by a scale as conventionally employed is limited.
For example, in order to obtain a high current driving ability it is necessary to increase the channel width. The monolithic circuit uses the TFT having a channel width of 500 to 1,000 μm. However, if a higher current driving ability is required due to the increase in the number of pixels and the degree of gradation, it is difficult to further expand the channel width to 5 mm, 10 mm or the like from that the formation region of the peripheral circuit is limited.
On the other hand, it is desirable for the TFT used to control the pixel to obtain a clear image quality by increasing a charge retention ability. However, considering that the pixel region has a size of several hundreds μm square, it is impossible to increase the channel length to 50 μm, 100 μm or the like in order to decrease the leak current. As a result, since a scale of a matrix, a pitch and the number of pixels are largely limited in the conventional TFT monolithic type active matrix circuit, a displaying unit having a finer screen capable of obtaining a high quality image cannot be manufactured.
The above problems occur in not only the monolithic type active matrix circuit but also in other semiconductor circuits.